The present invention relates to methods and optical instrumentation for objectively measuring the aberrations of the human eye and specifically to an instrument capable of measuring not only the focus (spherical) and astigmatism (cylindrical) characteristics and aberrations of a person""s eye but also all of the lower and higher order optical aberrations that are derived from a measured wavefront utilizing a wavefront curvature sensor.
Measuring the aberrations of an optical system, including a human eye, is an important part of working with any optical system. Existing methods of measurement include various interferometric techniques, the Shack-Hartman wavefront sensor, and various systems involving the projection of patterns through the optical system. These systems are typically complex and expensive and most require access to the focal plane.
The human eye, although comprised of only a few optical components, may manifest a wide variety of optical aberrations that vary from person to person and over time. These aberrations may result from surface contour shape, lens thickness factors, axial alignment of refractive surfaces, axial length of the eye, and even localized refractive index variations. The fact that the human eye possesses optical aberrations has been known for centuries. Nevertheless, the measurement and characterization of these aberrations, primarily the monochromatic aberrations, has remained a problem and has fostered much research in physiological optics over the years. Finding the proper prescription, even in modern times, has been primarily based on the subjective responses to the viewing of eye charts by the person being tested, whereas recent advances in corrective methods have emphasized objective measurement of these aberrations.
Current methods of optical correction of the human eye to allow clear vision include spectacles, contact and intraocular lenses, and refractive surgery. Spectacles, the most commonly used method, only allow correction of sphere (defocus) and regular or symmetric cylinder (astigmatism). None of the present methods allow for correction of other aberrations and thus do not maximize the optical potential of the visual system, leaving the images generated to be less than optimal. In addition, not only is the view out of the eye not optimal but so is the view in. Thus, the examination of the eye""s interior is also limited by these aberrations, and in some clinical situations, is severely handicapped.
Recently, great interest in this area has been kindled by the development of laser technologies, such as the excimer laser, whereby the refractive or optical errors of the eye, such as myopia (nearsightedness), hyperopia (farsightedness) and astigmatism, can be corrected by laser abelation or sculpting of the cornea. Such treatment creates a new corneal contour, or curvature, designed such that the image becomes clearly focused on the retina of the eye. Many degrees of myopia and hyperopia, with or without astigmatism, and astigmatism alone can now be corrected by such laser corneal surgery.
Although the clinical results of such surgery are good, it has been postulated, on the basis of experiment, that improved results could be obtained if the surgery were customized fully to correct all the optical aberrations of the eye, not just the sphere and cylinder. Super vision at the diffraction limit set by the aperture is possible. This could be accomplished by a computer-directed small spot scanning laser and sophisticated algorithm that takes into consideration all the aberrations of the eye. Also, many subject""s have irregularly shaped corneas, not currently treatable. In addition, the asphericity of the modified cornea is often significantly increased. Clinical studies have indicated that current autorefractors, when used to determine the refraction, or optical prescription, of surgically modified eyes may provide less reliable data in such cases. Even in normal eyes, their accuracy is such that the information cannot be routinely relied upon but must be verified by further subjective testing.
It is apparent that a complete diagnosis and understanding of the eye""s optical function, as the organic optical instrument, is currently very limited. A full evaluation should provide a complete description of the optical characteristics and aberrations in a quantitative format. Only then can there arise the possibility of correcting the abnormalities.
Theoretically, light arriving at the eye from a point source at infinity arrives in the form of a plane (flat) wave, whereas light from closer objects provide a wave with a convex spherical shape. This wave, in an ideal eye, would be focused as a discrete point limited only by diffraction on the retina of the eye. However, because of the optical aberrations of the eye, a degraded or blurred image is created on the retina. This concept can be appreciated in the reverse direction with resultant utility.
A plane wave, directed into the eye, would form a spot on the retina. In reverse this spot scatters light which escapes through the same optical path from which it came in. Because this light originates from a scattering process the incoming wavefront information is lost, resulting in a new source which originates from the back of the eye. This emergent wavefront now processes only the aberrations of the eye on a single pass. The present inventors have discovered that the distorted shape of this source, caused by incoming aberrations, can uniquely be eliminated with the differential curvature wavefront sensing method. Measurement and characterization of this wavefront allows one to describe the aberrations of the eye mathematically. Presently, some of these concepts are taken advantage of in ground-based telescope systems that are typically coupled with adaptive optical elements in a closed loop system. They can rapidly neutralize the wavefront aberrations induced by atmospheric turbulence and produce images that are limited only by diffraction and the aperture of the telescope.
Unfortunately, current subjective clinical methodology and instrumentation, such as the phoropter and objective devices such as autorefractors, do not avail themselves of this understanding and are based on concepts and techniques that restrict measurements to defocus and astigmatism only. During the past decade, this limitation has been appreciated and devices called corneal topographers, utilizing images reflected by the cornea, have been developed to obtain more optical information about the eye. However, they gather optical information about only one surface in the eye""s refractive system and reveal nothing about the system as a whole.
A number of investigators have attempted or suggested means whereby the wavefront, either explicitly or implicitly, was recognized as an entity to be captured and determined. These studies were interested primarily in determining the monochromatic aberrations of the eye rather than the development of autorefractor-like devices for routine clinical use or methods of correction.
A number of approaches have been taken to measure the monochromatic aberrations of the eye. Some used projecting rays or patterns of light into the eye and analysis of the images by subjective or objective means. Initially this work, such as present by M. S. Smirnov (xe2x80x9cMeasurement of the Wave Aberration of the Human Eyexe2x80x9d, Biophysics, 1961; 6:776-94) was carried out using subjective sequential subject testing, which was inaccurate and time consuming. More studies, however, have been performed using a modification of the principle first presented by Tscherning in 1894. One approach employed a device called the crossed cylinder aberroscope (Howland B and Howland H C: Subjective measurement of high-order aberrations of the eye. Science 1976; 193:580-02 and Howland H C and Howland B: xe2x80x9cA subjective method for the measurement of monochromatic aberrations of the eyexe2x80x9d, J. Opt Soc. Am 1977; 67(11): 1508-1518). Initially, this device was used in a subjective fashion whereby a drawing was made of an object by the test subject and later analyzed mathematically by computer to calculate the wavefront. This allowed for the mathematical characterization of the wavefront in mathematical terms, such as a Taylor series expansion or Zernike polynomials, and it was determined that the aberrations were dominated by third-order Taylor terms. Later, this method was converted to an objective approach by Walsh et al.
In the objective aberroscope method a point source of light is viewed through a grid placed close to the eye (Walsh et al., xe2x80x9cObjective Technique for the Determination of Monochromatic Aberrations of the Human Eyexe2x80x9d, J. Opt. Soc. Am. A., 1984; Vol. 1, No. 9, pp. 987-992 and Walsh G, Chairman W N: xe2x80x9cMeasurement of the axial wavefront aberration of the human eyexe2x80x9d, Opthal Physiol Opt, 1985, 5:23-31). This results in an aberrated image of the grid on the retina that can be photographed and analyzed by ray tracing. Although multiple points at the grid line intersections can be captured at the same time, it has been found that diffraction effects prevent sampling the papillary area at intervals much less than 1 mm which limit the determination of fine detail (Chairman W N: xe2x80x9cWavefront Aberration of the Eye: A Reviewxe2x80x9d, Optometry and Vision Science 1991; 68(3): 574-583). Other drawbacks were the lack of a rapid means of analysis and the faulty assumption that the aberrations could be characterized by terms of only up to the fourth order. The former problem has been improved upon with more computerized versions (Atchinson D A, Collins M J, Wildsoet C F, Christensen J. Waterworth M D: xe2x80x9cMeasurement of monochromatic ocular aberrations of human eyes as a function of accommodation by the Howland aberroscope techniquexe2x80x9d, Vision Res 1995; 35(3): 313-23 and Cox M J and Walsh G: xe2x80x9cReliability and validity studies of a new computer-assisted crossed-cylinder aberroscopexe2x80x9d, Optom Vis Sci 1997; 74 (7): 570-80).
Several investigators attempted other objective methods, whereby the wavefront was determined from the point-spread function data using a hybrid optical-digital instrument (Artal P, Santamarfa J. Bescos J: xe2x80x9cRetrieval of wave aberration of human eyes from actual point-spread-function dataxe2x80x9d, J Opt Soc Am 1988; 5(8); 1201-6).
Another objective approach was based on a modified Foucault knife-edge method as a double-pass ophthalmoscopic method and allowed wavefront aberrations to be inferred from two flash photographs obtained with the knife-edge oriented in orthogonal directions. This demonstrated significant irregular components (Berny F and Slansky S: xe2x80x9cWavefront determination resulting from Foucault tests applied to the human eye and visual instrumentsxe2x80x9d, Optical Instruments and techniques, Dickson J H (ed), London, Oriel, 1969, 375-85).
Howland used an approach whereby variations of focus across the natural pupil by employing a small artificial pupil and a telescope with an adjustable focus, and related the measured variations in focus to wave aberrations of the eye (Howland H C and Buettner J: xe2x80x9cComputing high order wave aberration coefficients from small variations of best focus for small artificial pupilsxe2x80x9d, Vision Res 1989; 29(8): 979-83).
The most recent direction taken in the measurement and correction of monochromatic aberrations of the eye involves the technologies of adaptive optics or deformable mirrors and wavefront sensors. The use of a deformable mirror has been proposed to assist in the neutralization of the wavefront error to improve the function of a confocal laser scanning ophthalmoscope for use with the human eye, and the method was to correct the low order aberration of astigmatism (Bille U.S. Pat. 4,838,679 and Dreher, Bille, and Weinreb, xe2x80x9cActive optical depth resolution improvement of the laser tomographic scannerxe2x80x9d, Applied Optics, 1989 Vol. 28, No. 4, pp. 804-808). In neither case, however, was a specific method to measure the aberrations developed or disclosed.
Others used a Hartmann-Shack wavefront sensor, developed and used in astronomy in conjunction with adaptive optics to neutralize atmospheric turbulence, to measure the eye""s aberrations (Williams et al. U.S. Pat. No. 5,777,719 and Liang, et al., xe2x80x9cObjective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensorxe2x80x9d, J. Opt. Soc. Am. A., July 1994 Vol. 11, No. 7, pp. 1-9). The sensor is an array of multiple lenslets, constructed of two identical layers of cylinders set 90 degrees apart that act as an array of spherical lenslets. The reflection of a beam of light incident onto the fovea is imaged by the lenslet array and analyzed by computer, deriving the wavefront emergent from the eye. An acknowledged limitation of the system was that only polynomials up to the fourth degree were used to represent the wavefront, which is considered inadequate (Williams U.S. Pat No. 5,777,719). Bille also proposed the combination of a wavefront sensor and an adaptive optical element but the details have never been disclosed (Bille et al.; xe2x80x9cScanning laser tomography of the living human eyexe2x80x9d; Noninvasive Diagnostic Techniques in Ophthalmology. Masters B R (ed), Springer-Verlag, 1990, pp. 528-47).
The first detailed description of a device that combined both a wavefront sensor and an adaptive optical element was disclosed by Williams, et al. U.S. Pat No. 5,777,719. The proposed instrument was primarily a retinal fundus imaging device that used the Hartmann-Shack wavefront sensor as the basis of obtaining the wavefront. The wavefront is expressed in terms of Zernike polynomials, which are then used to appropriately deform a mirror such that the eye""s aberrations could be neutralized or compensated for to provide high resolution retinal imaging or to provide supernormal vision while viewing with the assistance of the device. Disadvantages with this technique are the complexity, construction and cost of the Hartman-Shack wavefront sensor. Also, depending upon the magnitude of the aberrations, significant deviations of the wavefront at certain locations could be erroneously ascribed to the nearest lenslet whereas the signal arose from a location further away. In addition, the deformable mirror described is extremely complex and costly to construct. Although perhaps capable of determining and neutralizing the wavefront, the design does not describe how the device could be used as a common tool in clinical practice to determine the refraction of the eye in an economical way.
Another approach, called curvature wavefront sensing, is an old qualitative technique, which the present inventors have discovered can be made quantitative, with the aid of new technology and modern computers, for examining the human eye. It is inexpensive to implement and can determine a wide range or aberrations through a large tunable dynamic range. Curvature wavefront sensing has been employed in closed-loop adaptive optics systems in astronomy by one group for some years and has been reported on several publications where the optical principles involved are described (Roddier F: Curvature Sensing and Compensation: xe2x80x9cA new concept in adaptive opticsxe2x80x9d, Applied Optics, 1988, Vol. 27, pp. 1223-5; Roddier F: xe2x80x9cWavefront Sensing and the Irradiance Transport Equation. Applied Optics, 1990, Vol. 29 (10), pp. 1402-3; and Roddier C and Roddier F: xe2x80x9cNew Optical Testing Methods Developed at the University of Hawaii: Results of ground-based Telescopes and Hubble Space Telescopexe2x80x9d. SPIE, 1991, Vol. 1531, pp.37-3), which publications are incorporated herein by this reference for explanatory background.
It is a primary object of the present invention to provide both a method and an apparatus for accurately and quickly measuring the optical aberrations of the eye, including focus, astigmatism and higher order aberrations.
It is another object of the present invention to provide such an apparatus with a wavefront curvature sensor with the capability of measuring the optical aberrations of the eye.
It is a further object of the present invention to provide methods and apparatus for obtaining defocused pupil images that make diffractive effects symmetrical, and thus maximizes the accuracy of the wavefront sensor and the analysis of the aberrations of the human eye.
It is a still further object of the present invention to provide a method and apparatus for obtaining defocused images at high speed, thus allowing the information to be used for real time correction of the optical aberrations by several means.
Still a further object of the present invention is to provide a method and apparatus for tuning the dynamic range of the measurements taken with the wavefront sensor to suit the wavefront under consideration.
A still further object of the present invention is to provide a method and apparatus for minimizing strong images caused by the reflection from the ocular refracting surfaces of the eye in the measurement beam.
It is still another and further object of the present invention to provide a method and apparatus for measuring the pupil size and shape at the time of the wavefront measurement.
Other and more detailed objects and advantages of the present invention will appear to those skilled in the art from the following detailed description of the preferred embodiments in conjunction with the drawings.